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Leptin molecule. Computer model showing the structure of a molecule of the hormone leptin. Leptin is produced by adipose (fat) tissue. It interacts with receptors in the brain's hypothalamus to signal when a person is full. It also circulates in the blood at levels proportionate to body fat, which allows the brain to regulate appetite and metabolism. Mutations in the ob gene that codes for leptin are thought to be responsible for some forms of obesity.